Phase-locked loop (PLL) circuits are used in various different applications including, but not limited to, frequency tuners (e.g., satellite tuners) for selecting different television and/or radio channels. In a PLL circuit, a feedback loop is generally used to adjust frequency/phase of a voltage-controlled oscillator (VCO) output signal until the VCO output signal aligns with a reference clock signal.
The PLL circuit typically includes a charge pump circuit to provide voltage control signal Vc with sufficient charge for proper VCO operation. The charge pump circuit typically includes a transconductance cell, which may also be referred to as a transconductance amplifier, to generate current using input voltage signals, which are adjusted based on phase/frequency relationship between the reference clock and the VCO output signal.
The voltage control signal Vc output by the charge pump circuit is often limited by the transconductance capability of the transconductance cell. For example, the transconductance cell typically includes a current supply transistor that enters into triode region of operation as the voltage control signal Vc increases, resulting in abrupt change to the transconductance capabilities since the current supply transistor typically provides less current when operating in triode region. The sensitivity requirement of the VCO, Kvco, in any PLL design may be reduced when the VCO control voltage range of the charge pump circuit can be widened.
Therefore, it is desirable to provide a transconductance cell that is capable of transconductance over a wider range of input voltages.